U.S. patent application number 15/428150 was filed with the patent office on 2017-08-31 for method for controlling fuel cell system.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. The applicant listed for this patent is HONDA MOTOR CO., LTD.. Invention is credited to Nobuki KOIWA, Yuji MATSUMOTO, Osamu OGAMI, Kenji TARUYA.
Application Number | 20170250415 15/428150 |
Document ID | / |
Family ID | 59680166 |
Filed Date | 2017-08-31 |
United States Patent
Application |
20170250415 |
Kind Code |
A1 |
KOIWA; Nobuki ; et
al. |
August 31, 2017 |
METHOD FOR CONTROLLING FUEL CELL SYSTEM
Abstract
A method for controlling a fuel cell system including a fuel
cell, includes generating electric current via an electrochemical
reaction between a fuel gas and an oxidant gas by the fuel cell
including a solid polymer electrolyte membrane. An impedance of the
fuel cell is detected. It is determined whether the impedance
reaches a threshold impedance. The electric current generated by
the fuel cell is increased when the impedance is determined to
reach the threshold impedance.
Inventors: |
KOIWA; Nobuki; (Wako,
JP) ; OGAMI; Osamu; (Wako, JP) ; MATSUMOTO;
Yuji; (Wako, JP) ; TARUYA; Kenji; (Wako,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HONDA MOTOR CO., LTD. |
Tokyo |
|
JP |
|
|
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
59680166 |
Appl. No.: |
15/428150 |
Filed: |
February 9, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02J 7/00 20130101; H01M
8/04708 20130101; H01M 8/0491 20130101; H01M 8/04358 20130101; H01M
2250/20 20130101; H01M 8/04649 20130101; H01M 2008/1095 20130101;
Y02T 90/40 20130101; Y02E 60/50 20130101 |
International
Class: |
H01M 8/04858 20060101
H01M008/04858; H01M 8/0432 20060101 H01M008/0432; H02J 7/00
20060101 H02J007/00; H01M 8/04537 20060101 H01M008/04537 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2016 |
JP |
2016-035375 |
Claims
1. A method for controlling a fuel cell system including a fuel
cell, comprising: generating electric current via an
electrochemical reaction between a fuel gas and an oxidant gas by
the fuel cell including a solid polymer electrolyte membrane;
detecting an impedance of the fuel cell; determining whether the
impedance reaches a threshold impedance; and increasing the
electric current generated by the fuel cell when the impedance is
determined to reach the threshold impedance.
2. The method according to claim 1, wherein the electric current is
increased by a larger amount as the impedance increases.
3. A method for controlling a fuel cell system including a fuel
cell, comprising: generating electric current via an
electrochemical reaction between a fuel gas and an oxidant gas by
the fuel cell including a solid polymer electrolyte membrane;
cooling oxidant gas supplied to the fuel cell with a coolant;
determining whether a temperature of the coolant reaches a
threshold temperature; and increasing the electric current
generated by the fuel cell when the temperature of the coolant is
determined to reach the threshold temperature.
4. The method according to claim 3, wherein the electric current is
increased by a larger amount as the temperature of the coolant
increases.
5. A method for controlling a fuel cell system including a fuel
cell, comprising: generating electric current via an
electrochemical reaction between a fuel gas and an oxidant gas by
the fuel cell including a solid polymer electrolyte membrane;
detecting an impedance of the fuel cell; determining whether the
impedance reaches a threshold impedance; cooling oxidant gas
supplied to the fuel cell with a coolant; determining whether a
temperature of the coolant reaches a threshold temperature; and
increasing the output electric current generated by the fuel cell
when the impedance is determined to reach the threshold impedance
or when the temperature of the coolant is determined to reach the
threshold temperature.
6. The method according to claim 5, wherein, when the impedance
reaches the threshold impedance, the electric current is increased
by a larger amount as the impedance increases, and, when the
temperature of the coolant reaches the threshold temperature, the
electric current is increased by a larger amount as the temperature
of the coolant increases.
7. The method according to claim 1, wherein a battery is charged
with the electric current, and charging of the battery is finished
when the battery becomes fully charged.
8. The method according to claim 1, wherein an oxidant gas supply
mechanism supplies the oxidant gas at a lowest supply flow rate
when the fuel cell is performing idling electric power generation,
and the oxidant gas supply mechanism maintains the lowest supply
flow rate when increasing the electric current.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority under 35 U.S.C.
.sctn.119 to Japanese Patent Application No. 2016-035375, filed
Feb. 26, 2016, entitled "Method for Controlling Fuel Cell System."
The contents of this application are incorporated herein by
reference in their entirety.
BACKGROUND
[0002] 1. Field
[0003] The present disclosure relates to a method for controlling a
fuel cell system.
[0004] 2. Description of the Related Art
[0005] A solid polymer electrolyte fuel cell includes a membrane
electrode assembly (MEA), in which an anode electrode is disposed
on one surface of an electrolyte membrane, which is a
proton-conductive polymer ion-exchange membrane, and a cathode
electrode is disposed on the other surface of the electrolyte
membrane. The membrane electrode assembly and separators,
sandwiching the membrane electrode assembly, constitute a power
generation cell (unit cell). Typically, a predetermined number of
power generation cells are stacked and, for example, mounted in a
fuel cell vehicle (a fuel cell electric automobile or the like) as
a vehicle fuel cell stack.
[0006] The fuel cell generates electric power by causing an
electrochemical reaction between a fuel gas (such as hydrogen gas)
supplied to the anode electrode and an oxidant gas (such as
compressed air) supplied to the cathode electrode. During power
generation, protons are generated at the anode electrode, and the
protons are conducted through the electrolyte membrane and move to
the cathode electrode. At the cathode electrode, protons,
electrons, and oxygen in the oxidant gas react to generate
water.
[0007] The electrolyte membrane needs to be moist so that the
electrolyte membrane can conduct protons. For this purpose, the
generated water and moisture added to the oxidant gas keep the
electrolyte membrane moist. However, if the inside of the fuel cell
becomes excessively moist, water is retained in the fuel cell.
Because the retained water blocks small holes in the electrodes, it
becomes difficult for reactant gases to reach the electrodes, and
the rate of the electrochemical reaction is reduced.
[0008] In order to prevent this, Japanese Unexamined Patent
Application Publication No. 7-235324 describes that whether or not
the cathode electrode is excessively moist is determined on the
basis of the impedance of the fuel cell and that, when it is
determined that the cathode electrode is excessively moist, the
oxidant gas that has bypassed a humidifier is supplied to the
cathode electrode. In this case, in an attempt to remove water
droplets adhering to the cathode electrode, comparatively dry
oxidant gas is blown against the water droplets.
SUMMARY
[0009] According to a first aspect of the present invention, a
method for controlling a fuel cell system including a fuel cell,
includes generating electric current via an electrochemical
reaction between a fuel gas and an oxidant gas by the fuel cell
including a solid polymer electrolyte membrane. An impedance of the
fuel cell is detected. It is determined whether the impedance
reaches a threshold impedance. The electric current generated by
the fuel cell is increased when the impedance is determined to
reach the threshold impedance.
[0010] According to a second aspect of the present invention, a
method for controlling a fuel cell system including a fuel cell,
includes generating electric current via an electrochemical
reaction between a fuel gas and an oxidant gas by the fuel cell
including a solid polymer electrolyte membrane. Oxidant gas
supplied to the fuel cell is cooled with a coolant. It is
determined whether a temperature of the coolant reaches a threshold
temperature. The electric current generated by the fuel cell is
increased when the temperature of the coolant is determined to
reach the threshold temperature.
[0011] According to a third aspect of the present invention, a
method for controlling a fuel cell system including a fuel cell,
includes generating electric current via an electrochemical
reaction between a fuel gas and an oxidant gas by the fuel cell
including a solid polymer electrolyte membrane. An impedance of the
fuel cell is detected. It is determined whether the impedance
reaches a threshold impedance. Oxidant gas supplied to the fuel
cell is cooled with a coolant. It is determined whether a
temperature of the coolant reaches a threshold temperature. The
electric current generated by the fuel cell is increased when the
impedance is determined to reach the threshold impedance or when
the temperature of the coolant is determined to reach the threshold
temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings.
[0013] FIG. 1 is a schematic diagram of a fuel cell system to which
a method according to an embodiment of the present disclosure is
applied.
[0014] FIG. 2 is flowchart to explain the method according to the
embodiment of the present disclosure.
[0015] FIG. 3 is flowchart to explain the method according to the
embodiment of the present disclosure.
[0016] FIG. 4 is flowchart to explain the method according to the
embodiment of the present disclosure.
DESCRIPTION OF THE EMBODIMENTS
[0017] The embodiments will now be described with reference to the
accompanying drawings, wherein like reference numerals designate
corresponding or identical elements throughout the drawings.
[0018] Hereinafter, an embodiment of a method for controlling a
fuel cell system according to the present disclosure will be
described in detail with reference to the drawings. In the present
embodiment, an example in which the fuel cell system is mounted in
a fuel cell vehicle, such as a fuel cell electric automobile, is
described.
[0019] First, a fuel cell system 10 will be described with
reference to FIG. 1, which is a schematic diagram. The fuel cell
system 10 includes a fuel cell stack 12 (fuel cell).
[0020] The fuel cell stack 12 includes a fuel gas supply device 14
that supplies a fuel gas, an oxidant gas supply device 16 that
supplies an oxidant gas, and a coolant supply device 18 that
supplies a coolant. In the present embodiment, hydrogen gas is used
as the fuel gas, and compressed air is used as the oxidant gas. The
fuel cell system 10 further includes a battery 20 (storage battery)
that is an energy storage device, a controller 22 that is a system
control device, and an impedance measuring unit 23.
[0021] In FIG. 1, for ease of understanding, the controller 22 and
the impedance measuring unit 23 are independently illustrated. In
reality, however, the impedance measuring unit 23 is a part of the
controller 22. As described below, the impedance measuring unit 23
determines whether a solid polymer electrolyte membrane 32 is
likely to become dry or likely to become moist on the basis of the
impedance value of the fuel cell stack 12 (power generation cells
24).
[0022] The fuel cell stack 12 includes a plurality of power
generation cells 24 that are stacked in the direction of arrow A (a
horizontal direction or a vertical direction) in FIG. 1. In each of
the power generation cells 24, a membrane electrode assembly 26 is
sandwiched between a first separator 28 and a second separator 30.
The first separator 28 and the second separator 30 are made of
metal or carbon.
[0023] The membrane electrode assembly 26 includes, for example,
the solid polymer electrolyte membrane 32, which is a thin
water-soaked perfluorosulfonic acid membrane, and an anode
electrode 34 and a cathode electrode 36 that sandwich the solid
polymer electrolyte membrane 32. Instead of a fluoropolymer
electrolyte, a hydrocarbon (HC) electrolyte may be used as the
solid polymer electrolyte membrane 32.
[0024] A hydrogen gas channel 38 (fuel gas channel), for supplying
hydrogen gas to the anode electrode 34, is formed on a surface of
the first separator 28 facing the membrane electrode assembly 26.
An air channel 40, for supplying air to the cathode electrode 36,
is formed on a surface of the second separator 30 facing the
membrane electrode assembly 26. A coolant channel 42, through which
a coolant flows, is formed between the first separator 28 and the
second separator 30 that are adjacent to each other.
[0025] The fuel cell stack 12 has a hydrogen gas inlet 44a, a
hydrogen gas outlet 44b, an air inlet 46a, an air outlet 46b, a
coolant inlet 48a, and a coolant outlet 48b. The hydrogen gas inlet
44a extends through the power generation cells 24 in the stacking
direction and is connected to the supply side of the hydrogen gas
channel 38. Likewise, the hydrogen gas outlet 44b extends through
the power generation cells 24 in the stacking direction and is
connected to the discharge side of the hydrogen gas channel 38. The
hydrogen gas channel 38, the hydrogen gas inlet 44a, and the
hydrogen gas outlet 44b constitute an anode channel.
[0026] Likewise, the air inlet 46a extends through the power
generation cells 24 in the stacking direction and is connected to
the supply side of the air channel 40. The air outlet 46b extends
through the power generation cells 24 in the stacking direction and
is connected to the discharge side of the air channel 40. The air
channel 40, the air inlet 46a, and the air outlet 46b constitute a
cathode channel.
[0027] The coolant inlet 48a extends through the power generation
cells 24 in the stacking direction and is connected to the supply
side of the coolant channel 42. The coolant outlet 48b extends
through the power generation cells 24 in the stacking direction and
is connected to the discharge side of the coolant channel 42.
[0028] The fuel gas supply device 14 includes a hydrogen tank 50
that stores high-pressure hydrogen gas. The hydrogen tank 50 is
connected to the hydrogen gas inlet 44a of the fuel cell stack 12
through a hydrogen gas supply passage 51 (fuel gas supply channel).
The hydrogen gas is supplied to the fuel cell stack 12 through the
hydrogen gas supply passage 51.
[0029] The hydrogen gas supply passage 51 includes a main channel
52 and a hydrogen bypass channel 53. In the main channel 52, a main
injector 54 and an ejector 56 are arranged in series. The hydrogen
bypass channel 53 branches off from the main channel 52 at a
position upstream of the main injector 54 and is joined to the main
channel 52 at a positon downstream of the ejector 56. That is, the
hydrogen bypass channel 53 bypasses the main injector 54 and the
ejector 56.
[0030] A BP (bypass) injector 60 is disposed in the hydrogen bypass
channel 53. The BP injector 60 is a sub-injector that is used to
supply a high-concentration hydrogen gas when high-load power
generation is required for the fuel cell stack 12. The main
injector 54 is a main injector that is mainly used for a normal
operation of the fuel cell stack 12 (when normally generating
electric power).
[0031] A hydrogen gas discharge passage 62 (anode off-gas pipe) is
connected to the hydrogen gas outlet 44b of the fuel cell stack 12.
Exhaust hydrogen gas (anode off-gas), which is hydrogen gas that
has at least partially been used in the anode electrode 34, is
discharged from the fuel cell stack 12 through the hydrogen gas
discharge passage 62.
[0032] A gas-liquid separator 64 is disposed in the hydrogen gas
discharge passage 62. A hydrogen circulation channel 66 branches
off from a position downstream of the gas-liquid separator 64, and
a downstream portion of the hydrogen circulation channel 66 is
connected to the ejector 56. A hydrogen pump 68 is disposed in the
hydrogen circulation channel 66. In particular, when activated, the
hydrogen pump 68 causes the anode off-gas, which has been
discharged to the hydrogen gas discharge passage 62, to circulate
to the hydrogen gas supply passage 51 through the hydrogen
circulation channel 66 and the ejector 56.
[0033] One end of a purge channel 70 is connected to the hydrogen
gas discharge passage 62, and a purge valve 72 is disposed in the
purge channel 70. One end of a drain channel 74, for draining a
fluid mainly including a liquid component, is connected to a bottom
portion of the gas-liquid separator 64. A drain valve 76 is
disposed in the drain channel 74. The fuel gas supply device 14
includes a pressure sensor 77 for detecting hydrogen gas pressure
in the anode channel. The pressure sensor 77 is disposed, for
example, in the hydrogen gas supply passage 51 at a position near
the hydrogen gas inlet 44a. The pressure sensor 77 sends a
detection signal to the controller 22.
[0034] The oxidant gas supply device 16 includes an air pump 79,
which is disposed in an air supply passage 78 (oxidant gas supply
channel) and which corresponds to an oxidant gas supply mechanism.
The air pump 79 is a compressor that includes a motor, which
corresponds to a rotational driving unit, and that compresses and
supplies air. That is, the air pump 79 compresses air as the motor
rotates and supplies the compressed air to the fuel cell stack 12
through the air supply passage 78.
[0035] Detailed description of the air pump 79 will be omitted,
because an air pump having the aforementioned structure is
known.
[0036] The oxidant gas supply device 16 further includes a
water-cooling intercooler 81, which is disposed in the air supply
passage 78. In FIG. 1, the intercooler is represented as "I/C".
[0037] A drive train, which includes a driving motor and the like
for rotating driving wheels (which are not shown), is mounted in a
fuel cell vehicle. Coiling water, which is a coolant for cooling
the drive train, flows through the water-cooling intercooler 81.
The temperature of compressed air decreases by exchanging heat with
the cooling water. As can be understood from above, the
water-cooling intercooler 81 is a cooler for decreasing the
temperature of compressed air.
[0038] A temperature sensor 82 for detecting the temperature of the
cooling water is disposed at a cooling water outlet of the
water-cooling intercooler 81. The temperature sensor 82 sends a
signal of information regarding the temperature of the cooling
water to the controller 22.
[0039] The air supply passage 78 is located downstream of the air
pump 79 and connected to the air inlet 46a of the fuel cell stack
12, and a supply on-off valve 83a (inlet shut-off valve) and a
humidifier 84 are disposed between the air pump 79 and the air
inlet 46a. A bypass supply passage 86, which bypasses the
humidifier 84, is connected to the air supply passage 78. An on-off
valve 88 is disposed in the bypass supply passage 86.
[0040] An air discharge passage 90 (cathode off-gas discharge
passage) is connected to the air outlet 46b of the fuel cell stack
12. Exhaust compressed air (cathode off-gas), which is compressed
air that has at least partially been used in the cathode electrode
36, is discharged from the fuel cell stack 12 through the air
discharge passage 90.
[0041] The oxidant gas supply device 16 includes a pressure sensor
91 for detecting the pressure of compressed air in the cathode
channel. The pressure sensor 91 is disposed, for example, in the
air discharge passage 90 at a position near the air outlet 46b. The
pressure sensor 91 sends a detection signal of the detected
pressure to the controller 22.
[0042] The humidifier 84 is disposed downstream of the air
discharge passage 90. Therefore, the humidifier 84 exchanges water
and heat between the compressed air supplied from the air pump 79
and the cathode off-gas. A discharge on-off valve 83b (outlet
shut-off valve) and a back-pressure valve 92 are disposed in the
air discharge passage 90 at positions downstream of the humidifier
84. The other end of the purge channel 70 and the other end of the
drain channel 74 are connected and joined to a downstream portion
of the air discharge passage 90, thereby forming a diluting
portion.
[0043] The back-pressure valve 92 is pressure adjusting valve that
is used to control the pressure of compressed air supplied to the
cathode channel. That is, the internal pressure of the cathode
electrode 36 is adjusted by the back-pressure valve 92.
[0044] Two ends of an air bypass channel 94 are respectively
connected to the air supply passage 78 and the air discharge
passage 90 at a position upstream of the supply on-off valve 83a
and at a position downstream of the discharge on-off valve 83b and
downstream of the back-pressure valve 92. A BP-flow-rate adjusting
valve 96, which is used to adjust the flow rate of air that flows
through the air bypass channel 94, is disposed in the air bypass
channel 94.
[0045] An air circulation channel 98 is connected to the air supply
passage 78 and the air discharge passage 90 respectively at a
position downstream of the supply on-off valve 83a and at a
position upstream of the discharge on-off valve 83b. A circulation
pump 100 is disposed in the air circulation channel 98. The
circulation pump 100 causes discharge air, which has been
discharged to the air discharge passage 90, to circulate to the air
supply passage 78 through the air circulation channel 98.
[0046] The coolant supply device 18 includes a coolant supply
passage 102 that is connected to the coolant inlet 48a of the fuel
cell stack 12, and a water pump 104 is disposed in the coolant
supply passage 102. The coolant supply passage 102 is connected to
a radiator 106; and a coolant discharge passage 108, which is
connected to the coolant outlet 48b, is connected to the radiator
106.
[0047] Next, using flowcharts of FIGS. 2-4, a method for
controlling the fuel cell system 10 according to the present
embodiment will be described in relation to the operation of the
fuel cell system 10.
[0048] When starting a fuel cell vehicle including the fuel cell
system 10, the ignition is turned on and the fuel cell stack 12 is
activated. At this time, the controller 22 sends an instruction
signal for opening the main injector 54 to supply hydrogen gas from
the fuel gas supply device 14 to the anode channel. Thus, a
predetermined amount of hydrogen gas is supplied from the hydrogen
tank 50 to the hydrogen gas supply passage 51, the hydrogen gas
flows through the main injector 54 and the ejector 56 of the main
channel 52, and the hydrogen gas is supplied to the hydrogen gas
inlet 44a of the fuel cell stack 12.
[0049] The hydrogen gas flows from the hydrogen gas inlet 44a into
the hydrogen gas channel 38 and moves along the hydrogen gas
channel 38. Thus, the hydrogen gas is supplied to the anode
electrode 34 of the membrane electrode assembly 26.
[0050] Moreover, the controller 22 sends an instruction signal for
activating the air pump 79 to supply compressed air from the
oxidant gas supply device 16. Accordingly, the air pump 79 rotates
to compress air and to feed the compressed air to the air supply
passage 78. The compressed air is moistened while passing through
the humidifier 84 and then supplied to the air inlet 46a of the
fuel cell stack 12. The compressed air flows from the air inlet 46a
into the air channel 40 and then moves along the air channel 40,
and thereby the compressed air is supplied to the cathode electrode
36 of the membrane electrode assembly 26.
[0051] Accordingly, in the membrane electrode assembly 26, the
hydrogen gas supplied to the anode electrode 34 and oxygen in the
compressed air supplied to the cathode electrode 36 cause an
electrochemical reaction in the electrode catalyst layers and
electric power is generated (step S1 in FIG. 2). A part of moisture
added to the compressed air by the humidifier 84 permeates from the
cathode electrode 36 into the solid polymer electrolyte membrane 32
and reaches the anode electrode 34.
[0052] In the coolant supply device 18, the water pump 104 operates
to supply a coolant, such as pure water, ethylene glycol, oil, or
the like from the coolant supply passage 102 to the coolant inlet
48a of the fuel cell stack 12. The coolant flows along the coolant
channel 42, cools the power generation cells 24, and then is
discharged from the coolant outlet 48b to the coolant discharge
passage 108.
[0053] The hydrogen gas supplied to the anode electrode 34 and
partially consumed is discharged from the hydrogen gas outlet 44b
to the hydrogen gas discharge passage 62 as anode off-gas. At this
time, the anode off-gas carries moisture (water) that has reached
the anode electrode 34 as described above. That is, the anode
off-gas that flows into the gas-liquid separator 64 is a moist gas
including water.
[0054] The gas-liquid separator 64 separates most of water in the
anode off-gas. The liquid (water) is discharged from the drain
channel 74 when the drain valve 76 is opened. The anode off-gas,
from which water has been separated but in which a small amount of
mist remains, flows from the hydrogen gas discharge passage 62 into
the hydrogen circulation channel 66 as the hydrogen pump 68
operates. Then, the anode off-gas is sucked into the ejector 56
from the hydrogen circulation channel 66, joins a flow of new
hydrogen gas that has passed through the main injector 54, and is
supplied to the anode channel from the hydrogen gas supply passage
51. Thus, the anode off-gas is circulated and supplied to the fuel
cell stack 12.
[0055] The anode off-gas that has been discharged to the hydrogen
gas discharge passage 62 is discharged (purged), as necessary, to
the outside as the purge valve 72 is opened.
[0056] Likewise, compressed air that has been supplied to the
cathode electrode 36 and partially consumed is discharged from the
air outlet 46b to the air discharge passage 90 as cathode off-gas.
The cathode off-gas flows through the humidifier 84 to humidify new
compressed air supplied from the air supply passage 78, the
pressure of the cathode off-gas is adjusted to a set pressure of
the back-pressure valve 92, and the cathode off-gas is discharged
to the diluting portion. The cathode off-gas, which has been
discharged to the air discharge passage 90, is supplied from the
air circulation channel 98 to the air supply passage 78 as the
circulation pump 100 operates. In this case, the cathode off-gas is
also circulated and supplied to the fuel cell stack 12.
[0057] While the fuel cell stack 12 is being operated as described
above, the impedance measuring unit 23 measures the impedance of
the fuel cell stack 12, to be more specific, the impedance of the
power generation cells 24 (step S2 in FIG. 2). Moreover, the
temperature sensor 82 detects (measures) the temperature of cooling
water that has flowed through the water-cooling intercooler 81. The
controller 22 constantly acquires information regarding the
impedance and information regarding the temperature of cooling
water.
[0058] For example, when the fuel cell vehicle is idling, a load on
the fuel cell stack 12, that is, a required output electric current
is low. In this case, the required air flow rate (required oxidant
gas flow rate) of the fuel cell stack 12 is also low. Therefore,
the controller 22 sends an instruction signal for "decreasing the
rotation speed of the motor" to the air pump 79. When the air pump
79 receives the signal, the rotation speed of the motor decreases
and the supply flow rate of compressed air decreases. In
particular, in an idling state, the rotation speed of the motor
becomes the lowest rotation speed, and the supply flow rate of
compressed air becomes the lowest supply flow rate.
[0059] Therefore, the amount of water generated in the fuel cell
stack 12 decreases. Due to the decrease of the generated water, the
amount of moisture added to the solid polymer electrolyte membrane
32 decreases, and proton conduction decreases and the impedance of
the fuel cell stack 12 (the power generation cells 24)
increases.
[0060] If the impedance increases excessively, the solid polymer
electrolyte membrane 32 becomes excessively dry and it becomes
difficult to add moisture to the solid polymer electrolyte membrane
32 again. Thus, in the present embodiment, the controller 22
determines that the solid polymer electrolyte membrane 32 is
extremely dry when the impedance of the fuel cell stack 12 becomes
a predetermined threshold, and performs predetermined control (step
S3 in FIG. 2).
[0061] That is, the controller 22 increases the output electric
current of the fuel cell stack 12 (step S4 in FIG. 2). In response
to this instruction, in the fuel cell stack 12, the electrochemical
reaction in the membrane electrode assembly 26 is accelerated, and
the reaction rate of the electrochemical reaction increases.
Accordingly, in the cathode electrode 36, the amount of water
generated due to the electrochemical reaction increases.
[0062] The generated water adds moisture to the solid polymer
electrolyte membrane 32. Therefore, the solid polymer electrolyte
membrane 32 is kept moist. As the impedance increases, the solid
polymer electrolyte membrane 32 becomes more likely to be dry.
Accordingly, when the impedance is high, preferably, the output
electric current is increased. This is because, by doing so, the
electrochemical reactions are accelerated further and the amount of
generated water increases further, and therefore the solid polymer
electrolyte membrane 32 is easily kept moist.
[0063] The increased output electric current is used, for example,
to charge the battery 20, and thus consumed. The battery 20 is
overcharged if the battery 20 is charged after having been fully
charged. To avoid this, after the battery 20 has been fully
charged, supply of an electric current to the battery 20 is
finished.
[0064] In this case, preferably, the amount of increase in the
output electric current is within a range with which the air pump
79 can maintain the lowest supply flow rate. That is, for example,
if the air pump 79 has a function of supplying compressed air with
a flow rate exceeding the lowest supply flow rate when the electric
current is 25 A or larger, the output electric current can be
increased to about 25 A.
[0065] In other words, in the above case, the air pump 79 supplies
compressed air at the lowest supply flow rate until the output
electric current becomes 25 A or larger. Therefore, by setting the
upper limit of the output electric current lower than 25A, an
increase in the supply flow rate of compressed air from the air
pump 79 can be avoided. Thus, the solid polymer electrolyte
membrane 32 is prevented from becoming dry due to an increase in
the amount of compressed air that contacts the cathode electrode
36. Moreover, because the operation amount of the oxidant gas
supply mechanism does not change, a user of the fuel cell vehicle
does not experience an unpleasant sensation.
[0066] Moreover, because an increase in the output electric current
can be managed as described above, the output of the fuel cell
stack 12 does not vary. Also for this reason, the driver is further
prevented from experiencing an unpleasant sensation.
[0067] After the impedance has sufficiently decreased, the
controller 22 makes the output electric current of the fuel cell
stack 12 to return to the original value. Subsequently, when the
impedance increases again and reaches the predetermined threshold,
the control is performed again.
[0068] Instead of the control described above, control based on the
temperature of cooling water that has flowed through the
water-cooling intercooler 81. This case will be described
below.
[0069] As described above, after step S1, cooling water for cooling
the drive train flows through the water-cooling intercooler 81. By
using the cooling water, high-temperature compressed air obtained
by the air pump 79 is cooled (step S11 in FIG. 3), and the
temperature of the compassed air decreases. In other words, the
compressed air is not sufficiently cooled when the temperature of
the cooling water is high, and the compressed air having a
comparatively high temperature flows through the air supply passage
78.
[0070] The compressed air flows into the humidifier 84, and the
humidifier 84 exchanges heat with the cathode off-gas. In this
case, as the temperature of compressed air increases, the
temperature difference between the compressed air and the cathode
off-gas is low, and therefore the amount of moisture added is
small. That is, in this case, compressed air, to which a sufficient
amount of moisture has not been added, is supplied to the cathode
electrode 36. As a result, the solid polymer electrolyte membrane
32 is likely to become dry.
[0071] In this case, the controller 22 determines whether or not
the temperature of the coolant, which is measured by using a
temperature sensor 82 disposed at a cooling water outlet of the
water-cooling intercooler 81, has reached a predetermined
threshold. When the temperature of the coolant becomes the
predetermined threshold (step S12 in FIG. 3), the controller 22
determines that the solid polymer electrolyte membrane 32 is
extremely dry, and performs predetermined control. That is, in the
same way as described above, the controller 22 sends an instruction
for increasing the output electric current of the fuel cell stack
12 (step S4 in FIG. 3). In response to this instruction, the
electrochemical reaction in the membrane electrode assembly 26 is
accelerated.
[0072] As a result, the amount of generated water increases. The
generated water adds moisture to the solid polymer electrolyte
membrane 32, and the solid polymer electrolyte membrane 32 is kept
moist.
[0073] As the temperature of the cooling water increases, the
temperature of compressed air that flows into the humidifier 84
increases, and the solid polymer electrolyte membrane 32 is more
likely to become dry. Accordingly, when the temperature of the
cooling water is high, preferably, the output electric current is
increased. This is because, also in this case, the electrochemical
reaction is further accelerated, the amount of generated water
further increases, and therefore the solid polymer electrolyte
membrane 32 can be kept moist easily.
[0074] The increased output electric current may be consumed by
being stored in the battery 20 in the same way as described
above.
[0075] After the temperature of the cooling water has sufficiently
decreased, the controller 22 returns the output electric current
required for the fuel cell stack 12 to the original level.
Subsequently, if the temperature of the cooling water increases
again and reaches the predetermined threshold, the above control is
performed again.
[0076] The two control methods described above may be used in
combination. In this case, after step S1, the output electric
current of the fuel cell stack 12 may be increased when the
impedance reaches a predetermined threshold or the temperature of
the cooling water reaches a predetermined threshold (steps S21 and
S4 in FIG. 4).
[0077] By performing control as described above, the solid polymer
electrolyte membrane 32 can be prevented from becoming excessively
dry.
[0078] The present disclosure is not limited to the embodiment
described above, which can be modified in various ways within the
spirit and scope of the present disclosure.
[0079] For example, the use of the fuel cell system 10 is not
limited to a vehicle. The fuel cell system 10 may be a stationary
system.
[0080] According to one aspect of the disclosure, provides a method
for controlling a fuel cell system including a fuel cell that
includes a solid polymer electrolyte membrane and that generates
electric power by causing an electrochemical reaction between a
fuel gas and an oxidant gas, a fuel gas supply device that supplies
the fuel gas into the fuel cell, an oxidant gas supply device
including an oxidant gas supply mechanism that supplies the oxidant
gas into the fuel cell, and an impedance measuring unit that
measures an impedance of the fuel cell. The method includes
increasing an output electric current of the fuel cell when the
impedance measured by the impedance measuring unit reaches a
predetermined threshold.
[0081] As the output electric current increases, the
electrochemical reaction in the electrodes of the fuel cell is
accelerated. As a result, the reaction rate increases and the
amount of generated water increases. Because the generated water
adds moisture to the solid polymer electrolyte membrane, the solid
polymer electrolyte membrane can be prevented from becoming
excessively dry. This is particularly effective when the solid
polymer electrolyte membrane is likely to become dry even in an
idling electric power generation state, in which the oxidant gas
supply mechanism supplies the oxidant gas at the lowest supply flow
rate.
[0082] As the impedance increases, the solid polymer electrolyte
membrane is more likely to become dry. Therefore, preferably, the
output electric current is increased by a large amount when the
impedance is high. In this case, the electrochemical reaction is
further accelerated and the amount of generated water further
increases, and therefore a sufficient amount of moisture can be
added to the solid polymer electrolyte membrane.
[0083] Instead of using the impedance, determination may be made by
using the temperature of a coolant for cooling the oxidant gas,
which flows through a cooler. Another aspect of the present
disclosure provides a method for controlling a fuel cell system
including a fuel cell that includes a solid polymer electrolyte
membrane and that generates electric power by causing an
electrochemical reaction between a fuel gas and an oxidant gas, a
fuel gas supply device that supplies the fuel gas into the fuel
cell, and an oxidant gas supply device including an oxidant gas
supply mechanism that supplies the oxidant gas into the fuel cell
and a cooler that cools the oxidant gas by using a coolant. The
method includes increasing an output electric current of the fuel
cell when a temperature of the coolant reaches a predetermined
threshold.
[0084] Also in this case, the electrochemical reaction in the
electrodes of the fuel cell is accelerated as the output electric
current increases. Accordingly, it is possible to prevent the solid
polymer electrolyte membrane from becoming excessively dry.
[0085] As the temperature of the coolant increases, the solid
polymer electrolyte membrane is more likely to become dry.
Therefore, the output electric current is increased by a large
amount when the temperature of the coolant is high. In this case,
as in the above case, the electrochemical reaction is further
accelerated and the amount of generated water further increases,
and therefore a sufficient amount of moisture can be added to the
solid polymer electrolyte membrane.
[0086] By using both of the impedance and the temperature of the
coolant for cooling the oxidant gas, determination may be made on
the basis of a condition on the impedance is satisfied or a
condition on the temperature. Another aspect of the present
disclosure provides a method for controlling a fuel cell system
including a fuel cell that includes a solid polymer electrolyte
membrane and that generates electric power by causing an
electrochemical reaction between a fuel gas and an oxidant gas, a
fuel gas supply device that supplies the fuel gas into the fuel
cell, an oxidant gas supply device including an oxidant gas supply
mechanism that supplies the oxidant gas into the fuel cell and a
cooler that cools the oxidant gas by using a coolant, and an
impedance measuring unit that measures an impedance of the fuel
cell. The method includes increasing an output electric current of
the fuel cell when the impedance measured by the impedance
measuring unit reaches a predetermined threshold or a temperature
of the coolant reaches a predetermined threshold.
[0087] In this case, when the solid polymer electrolyte membrane is
likely to become dry, control for increasing the amount of
generated water is performed. Accordingly, the solid polymer
electrolyte membrane can be more easily prevented from becoming
dry.
[0088] This control may be performed so that, when the impedance
reaches the predetermined threshold before the temperature of the
coolant reaches the predetermined threshold, the output electric
current is increased by a larger amount as the impedance increases.
Conversely, this control may be performed so that, when the
temperature of the coolant reaches the predetermined threshold
before the impedance reaches the predetermined threshold, the
output electric current is increased by a larger amount as the
temperature of the coolant increases.
[0089] The increased amount of the output electric current may be
used, for example, to charge a battery and consumed. If charging is
continued after the battery has been fully charged, the battery
becomes overcharged. Preferably, charging of the battery is
finished when the battery becomes fully charged.
[0090] In the above cases, the oxidant gas supply mechanism may
supply the oxidant gas at a lowest supply flow rate when the fuel
cell is performing idling electric power generation, and the
oxidant gas supply mechanism may maintain the lowest supply flow
rate when increasing the output electric current (idling electric
current) of the fuel cell. In other words, preferably, the amount
of increase in the output electric current is within a range of
electric current value with which the oxidant gas supply mechanism
maintains the lowest supply flow rate.
[0091] In this case, although the output electric current is
increased, the supply flow rate of the oxidant gas from the oxidant
gas supply mechanism is maintained at the lowest supply flow rate.
Accordingly, increase in the supply flow rate of the oxidant gas to
the cathode electrode is avoided, and therefore it is possible to
prevent the solid polymer electrolyte membrane from becoming dry.
Moreover, because the operation amount of the oxidant gas supply
mechanism does not change, a user can be prevented from
experiencing an unpleasant sensation.
[0092] With the present disclosure, information regarding the
impedance of the fuel cell is obtained and/or information regarding
the temperature of the coolant that cools the oxidant gas and that
flows through the cooler are/is obtained. Preferably, both of the
information regarding the impedance of the fuel cell and the
information regarding the temperature of the coolant are obtained.
The output electric current of the fuel cell stack is increased
when the impedance reaches a predetermined threshold, when the
temperature of the cooling water reaches a predetermined threshold,
or, preferably, when the impedance reaches a predetermined
threshold or the temperature of the cooling water reaches a
predetermined threshold. Therefore, the amount of generated water
increases, and moisture is added to the solid polymer electrolyte
membrane of the fuel cell. Accordingly, the solid polymer
electrolyte membrane can be prevented from becoming excessively
dry.
[0093] This control is particularly effective when idling electric
power generation, in which the oxidant gas supply mechanism
supplies the oxidant gas at the lowest supply flow rate, is being
performed and the solid polymer electrolyte membrane is likely to
become dry although the supply flow rate of the oxidant gas cannot
be reduced.
[0094] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *